1. Field of the Invention
This invention generally relates to a method and apparatus for making optical measurements and more specifically to a method and apparatus for measuring the topography of an eye particularly over the regions of the cornea and sclera.
2. Description of Related Art
Accurate knowledge of the surface shape or “topography” of a patient's eye is essential to a number of ophthalmic procedures, examinations and diagnoses of the eye, such as cataract procedures, Lasik procedures and the manufacture of properly fitting, optically functional, comfortable contact lens. In all these applications information about the topography of the cornea and sclera is particularly important. In regions where a lens contacts the eye, including regions under the eyelids, precise knowledge of eye topology greatly facilitates manufacturing the lens in such a way as to achieve optimal rotational orientation and patient comfort. However, measuring eye topology, and more specifically that of the cornea and sclera, is complicated by the steep curvature of the eye in the scleral region, eyelid obstruction of the upper and lower scleral regions, and random eye motion during the measurement procedure.
Prior art apparatus incorporating Optical Coherent Tomography (OCT) and Placido Rings technology and currently used to measure eye topology have various limitations with respect to acquiring measurements that can be used to construct an accurate model of an individual's eye. Placido Ring technology, for example, operates by projecting a series of concentric rings onto the surface of the eye and measuring ring placement and distortion to determine surface topology. However, this technology does not provide sufficient and accurate information due to surface irregularities along the rotational axis of the projected ring. Consequently information can be lost if the rings merge together due to irregularities in eye surface topology. Resolution is limited in the radial direction due to a small number of rings, on the order of 22, and surface topology cannot be measured under the eyelids. OCT operates by acquiring a temporal series of height profile cross sections of the eye scanned at different angular positions around an axis normal to the front surface of the pupil. To create a three-dimensional model of the eye multiple cross sections must be scanned without the eye moving and then accurately combined maintaining strict alignment between the scanned cross sections. Unfortunately, if the eye moves during or between successive sequential cross sectional measurements, the model will be inaccurate, because there is no way to reference each cross section to a fixed spatial reference on the patient's eye. In addition, cross sections are currently limited to approximately 16 mm in diameter due to the steep curvature of the sclera. A 16 mm diameter image is insufficient to incorporate the wide scleral contact lenses with diameters up to 22 mm. Moreover, OCT and Placido Ring scanners are not readily adapted to measure scleral topology under an individual's eyelids.
U.S. Pat. No. 5,493,109 to Wei et al. (hereinafter “Wei-109”) discloses OCT apparatus with an ophthalmologic surgical microscope. Automatic focusing is provided by driving a motorized internal focusing lens of the ophthalmologic surgical microscope with a signal output from the OCT apparatus. An embodiment of such a system includes: (a) optical coherence tomography apparatus; (b) a beam combiner for internally coupling output from the OCT apparatus into the ophthalmologic surgical microscope; and (c) a motor for moving an internal focusing lens of the ophthalmologic surgical microscope in response to a signal from the OCT apparatus.
U.S. Pat. No. 6,741,359 to Wei et al. (hereinafter “Wei-359”) discloses OCT apparatus and describes the particular methodology and limitations of the system described in Wei-109. Wei-359 discloses one embodiment of a scanner for a beam of scanning OCT radiation that includes: (a) a source of OCT radiation; (b) a scanner; and (c) scanning optics in which an image surface has a negative field curvature. As disclosed, Wei-109 is limited to scanning the corneal region of the eye. This system utilizes a large aperture and auto-focusing to meet the depth of field parameters of the cornea. As a result, and as described later, this process also affects the light collection efficiency of the system. In Wei-359 custom optics focuses a beam of OCT radiation in a curved arc that approximates that of the human cornea. The object is to confine OCT radiation to be parallel to the surface of the eye. Such a system produces depth profile information along user-programmed radial scan lines that traverse the diameter of the eye. As a practical matter such a system appears limited to measurements of the corneal region and unable to cover the corneal and entire sclera regions.
Recently attempts have been made to manufacture “scleral lenses” for individuals whose corneas were damaged or deformed by accidents, such as explosions, or who are diagnosed with Keratoconus, a disease that causes the central area of the cornea to thin and bulge outward. As conventional contact lens sit on the corneal surface, they are not appropriate for such individuals. However, scleral contact lenses rest on the sclera and not the deformed cornea. Consequently such sclera lenses have been used to restore vision to many patients. The lens works by creating a new optical surface that is raised above the damaged cornea. The gap between the back of the lens and corneal surface fills with the patient's own tears creating a pool of liquid tears that act as a liquid bandage to soothe the nerves on the corneal surface. The newly formed rigid front optical surface of the lens then focuses light through the patient's eye onto the back retina to restore vision. To achieve the best optical performance with optimal patient comfort, the scleral lens must perfectly match the shape, curvature, and topology of the patient's sclera, which is the bearing surface of the lens including the bearing regions under the eyelids.
Prior art measurement apparatus, such as OCT apparatus, is not an optimal choice for making such measurements due to their limited scanning diameter and area. That is, such prior art measurement systems cannot reach sufficiently far into the sclera where the lens contacts the eye. Moreover, a human eye often has a toric shape so the long and short axes not necessarily at 0° and 90°. Toric eye profile information also needs to be computed to achieve optimal fit. Prior art systems do not measure the toricity of the sclera or provide the scan orientation of the long and short toric axes.
This lack of measurement capability has limited the use of sclera lens. Currently, it is necessary to manufacture a set of trial lenses to allow a physician to determine the most comfortable lens in the set, much like finding the best shoe size that fits a customer's foot without even having a ruler to first measure foot size. As will be apparent, fitting scleral lenses is very time consuming, can only be performed by a few specially trained doctors and trained personnel, requires skilled personnel at all phases and is very expensive.